As seen in FIG. 1, a conventional boiling water reactor has a reactor pressure vessel 10 and a core shroud 12 arranged concentrically in the RPV with an annular region 8, commonly referred to as the "downcomer annulus", therebetween. The core shroud 12 is a stainless steel cylinder surrounding the nuclear fuel core comprising a plurality of fuel bundle assemblies (not shown). Each array of fuel bundle assemblies is supported at the top by a top guide and at the bottom by a core plate. During operation of the reactor, water is continuously recirculated down the downcomer annulus 8 and then up through the core. This flow is induced by a multiplicity of jet pumps located in the downcomer annulus and driven by recirculation pumps (not shown) outside the reactor pressure vessel.
The core shroud 12 comprises a shroud head flange 12a for supporting the shroud head 28; a circular cylindrical upper shroud wall 12b having a top end welded to shroud head flange 12a; an annular top guide support ring 12c welded to the bottom end of upper shroud wall 12b; a circular cylindrical middle shroud wall comprising three sections 12d, 12e and 12f welded in series, with a top end of section 12d being welded to top guide support ring 12c; and an annular core plate support ring 12g welded to the bottom end of middle shroud wall section 12f and to the top end of a lower shroud wall 12h. The entire shroud is supported by a shroud support 14, which is welded to the bottom of lower shroud wall 12h, and by annular shroud support plate 16, which is welded at its inner diameter to shroud support 14 and at its outer diameter to RPV 10.
In the event of a seismic disturbance, it is conceivable that the ground motion will be translated into lateral deflection relative to the reactor pressure vessel of those portions of the shroud located at elevations above shroud support plate 16. Such deflections would normally be limited by acceptably low stresses on the shroud and its weldments. However, if the shroud weld zones have failed due to stress corrosion cracking, there is the risk of misalignment and damage to the core and the control rod components, which would adversely affect control rod insertion and safe shutdown.
Stress corrosion cracking in the heat affected zone of any shroud girth seam welds diminishes the structural integrity of shroud 12, which vertically and horizontally supports the core top guide and the shroud head 22. In particular, a cracked shroud increases the risks posed by a loss-of-coolant accident (LOCA). During a LOCA, the loss of coolant from the reactor pressure vessel produces a loss of pressure above the shroud head 22 and an increase in pressure inside the shroud, i.e., underneath the shroud head. The result is an increased lifting force on the shroud head and on the upper portions of the shroud to which the shroud head is bolted. If the core shroud has fully cracked girth welds, the lifting forces produced during a LOCA could cause the shroud to separate along the areas of cracking, producing undesirable leaking of reactor coolant.
A known repair method for vertically restraining a weakened core shroud utilizes tensioned tie rods coupled to the shroud flange 12a and to the shroud support plate 16, as seen in FIG. 1. In addition, the shroud is restrained laterally by installation of wishbone springs which, along with the tie rod, are components of the shroud repair assembly. During repair of the shroud, a tie rod must be lifted from a horizontal position on the refueling floor to a vertical position suspended from a hoisting cable. The hoist is then transported to the azimuthal position whereat the shroud repair assembly is to be installed and then lowered into the downcomer annulus. Once the lower end of the tie rod has been vertically supported, the hoisting cable is disengaged from the upper end of the tie rod. The upper end of the tie rod is then ready to be anchored on the shroud flange. While the tie rod is suspended from the cable, it is critical that the tie rod, which weighs more than 1,000 pounds, not be dropped into the annulus. Such an accident could cause damage to the jet pump assemblies in the downcomer annulus. Also the operation to retrieve the dropped tie rod lengthens the duration of the shroud repair operation and, consequently, increases reactor downtime.